As is well known, the human visual system is trichromatic. All colors can be matched by a mixture of three primary colors: red, green, and blue. This trichromatic property of human color vision is a consequence of three different photoreceptor types in the retina: the R receptor (maximally sensitive to the red end of the spectrum), the G receptor (maximally sensitive to the middle, or green part of the spectrum), and the B receptor (maximally sensitive to the blue end of the spectrum). The ratios of red, green, and blue primaries in a color mixture that is made to match some arbitrary color is determined by the spectral absorption characteristics of the photopigments in the three types of receptors.
Approximately 8% of males and 0.2% of females have abnormal color vision due to a defect inherited on the X chromosome. One form of color vision defect, called anomalous trichromacy, is characterized by abnormal spectral absorption characteristics of the R receptor photopigment (protanomalous trichromat) or of the G receptor photopigment (deuteranomalous trichromat). Anomalous trichromats still require 3 primary colors in a mixture to perform color matching, however, the ratio of the primaries are anomalous in comparison to the normal population. A second form of color vision defect, called dichromacy, is characterized by an absence of the R receptor photopigment (protanope) or an absence of the G receptor photopigment (deuteranope). Protanopes and deuteranopes equire only two primary colors in a mixture to perform color matching.
X-linked color vision defects are diagnosed on the basis of tests that are standardized against color matching. In particular, a yellow light is matched by normal trichromats with a unique mixture of a red primary and a green primary. Relative to the normal match, protanomalous trichromats add too much red to the mixture and deuteranomalous trichromats add too much green to the mixture. Protanopes and deuteranopes match the red alone to the yellow, green alone to the yellow, and any ratio of red to green in a mixture to the yellow. The major difference between protanopes and deuteranopes is in their sensitivity to colored lights. Protanopes are very insensitive to red light, consequently red lights appear much dimmer than do yellow or green lights that look to be the same brightness for normals. Deuteranopes have the same sensitivity to colors as does the normal.
A prior art instrument which uses the above-stated principle for testing the color vision deficiency of an observer is give in Grolman, et al. U.S. Pat. No. 3,947,099. There an anomaloscope that uses a bipartite field having a spectrally pure yellow half and a mixture red and green half for testing the color vision of an observer is diclosed. However, the problem with this type of anomaloscope is that it is very difficult to use. For instance, in order to adjust the spectral red and green colors of the mixture on one half of a display circle to correspond with the spectrally pure yellow color on the other half of the display circle, the observer has to be both able to adjust the ratio of the red and green colors in the mixture and to adjust the brightness thereof so that it will look just like the spectral yellow color. Accordingly, since such anomaloscope is so difficult to use, most clinicians, in testing a color vision patient, would resort to the use of pseudoisochromatic plates, an example of which is given in Hardy, et al. U.S. Pat. No. 2,937,567. Briefly, the pseudoisochromatic plates are books that have printed patterns embedded in backgrounds of different colors. The printed patterns are in different hues of red and green colors. However, these hues are not calibrated. Thus, although the plates are very easy to use, since the patient observer only needs to look at the different plates and inform the clinician when he sees a figure, the plates do not have the precision, in terms of determining the type and degree of color vision a patient has, of an anomaloscope.
There exists, therefore, a need for an instrument that can provide for both easy operation and precise measurements of a patient observer's color vision.